Arsenic and cancer: effects of joint administration of arsenite and selenite on the genesis of mammary adenocarcinoma in inbred female C3H/St mice.

The joint administration of 2 ppm of arsenic as arsenite and of 2 ppm of selenium as selenite in the drinking water of inbred female C3H/St mice increases the incidence of spontaneous mammary adenocarcinoma significantly over the simultaneous controls and animals receiving 2 ppm As as arsenite or 2 ppm Se as selenite alone, indicating that arsenic abolishes the anticarcinogenic effect of selenium. Arsenite caused a significant increase of the tumor growth rates and raised the incidence of multiple tumors. The tumor growth rates in the As-Se group were also increased, but the incidence of multiple tumors was lower than in the 2 ppm As group.